If you are lucky and live in certain areas of the country, some grocery stores will double or even triple manufacturers' coupons. This is where the grocery store will match (usually with some limits) the price listed on the coupon offered by manufacturers. Times and days when double coupons are offered vary from store to store, so check with your local store for days that they offer them.

I didn't want to copy the whole list, but go to the link above to find your state.